Published papers

In this paper, we design a device that integrates a microchannel and optical waveguide core, and fabricate it using the Mosquito method. Then, the light output is detected from the core after transmitting through the microchannel.

Embedded optical waveguides with a circular core doped with europium (Eu)-aluminum (Al) polymer composite are fabricated using the Mosquito method, and the relationship between the core diameter and amplification gain is investigated. Further increase in gain is expected by setting up the amplification measurement system appropriately for waveguides with large core diameters.

In this paper, a polymer-based optical waveguide sensor is created to address several issues in optical fiber-based sensors. The unique feature of our optical waveguide is its 3-D core arranged design fabricated using the Mosquito method to which poly dimethyl siloxane (PDMS) is applied as the cladding for a flexible tactile sensor. The core monomer is dispensed into the PDMS monomer to have a 3D lattice structure. We demonstrate that the fabricated waveguide sensors exhibit a high repeatability of the pressure sensing. We find that the measurement range is from 0 MPa to 0.164 MPa when an organic inorganic hybrid resin, NP-C01-KO is used as the main core material. By examining the effect of the numerical aperture and the core size on the sensitivity, we confirm that bending loss could cause the output power variation when a load is applied to the sensor.

In this paper, we theoretically estimate the temperature dependence of coupling loss between single-mode (SM) polymer optical waveguides and general-purpose SM fiber (SMF), and discuss about the applicability of polymer optical waveguides to co-packaged optics (CPO). Although SM polymer optical waveguides are expected to be applied as an element of CPO, their small cores require precise mode-field adjustment with precise alignment of the cores. The mode-field and alignment positions of polymer waveguides are conce1ed to be affected by their operation temperature, particularly when integrated with Si photonics chips that could be a heat source. Simulation results show that the properties are maintained even under high-temperature operation.

Recent advancements in thin-film lithium niobate have led to the development of high-performance integrated electro-optic modulators, which are crucial for modern optical communication systems. These modulators offer tighter mode confinement, a smaller physical footprint, and reduced modulating voltages. This study presents a Mach-Zehnder modulator (MZM) on a silicon nitride-loaded lithium niobate platform using a few-mode waveguide structure. By harnessing the exceptional thermo-optic and electro-optic effects of LiNbO3, we design and simulate this modulator employing multilayer structures with the BeamPROP solver. The modulator has a length of 3.94 mm, a Vπ value of 0.96 V, and a transition temperature (Tg) of 80 °C at 1.55 µm. This proposed modulator exhibits a crosstalk of approximately-42 dB, an extinction ratio of approximately 24 dB, and a maximum transmission of-28 dB for the first-order phase shift. These findings demonstrate the significant potential of this modulator for deployment in high-speed optical communication systems, where maintaining thermal stability and optimizing energy efficiency are paramount.

We develop a unique optical connect structure with polymer waveguide fabricated by the Mosquito method for Co-Packaged Optics. We present the details of this optical connect structure and evaluation results of prototype waveguide.

In this paper, we propose a low-loss assembly of optical elements [vertical cavity surface emitting laser (VCSEL)/photodiode (PD)] and a 90°-bent graded-index (GI) core polymer optical waveguide for compact high-speed optical transceivers. The air gap between the 90°-bent GI-core waveguide and VCSEL/PD chips are filled with a high-index resin, and we demonstrate a reduction not only of the Fresnel reflection loss but also of the coupling loss due to the lower divergence angle of VCSEL and fiber output beams. In this paper, the core size and NA of the 90°-bent GI-core in the polymer waveguide are redesigned to match the specific launch conditions realized with the gap filled with resins, resulting in theoretical estimates of insertion losses as low as 0.82 dB and 1.38 dB for the waveguides on the transmitter and receiver sides, respectively. Such low insertion losses are obtained not by step-index core but GI core waveguides. After redesigning the structure, 90°-bent GI-core polymer waveguides with a bending radius of 1 mm are experimentally fabricated using the Mosquito method, and we experimentally demonstrate that the insertion loss of the waveguide can be reduced by about 4 dB when the 100-µm air gap is filled with an optical adhesive (na = 1.51). In addition, an error-free 25.78-Gb/s data transmission is successfully demonstrated over a 100-m MMF link using the fabricated VCSEL based transceivers with 90°-bent GI core polymer optical waveguides.

In this paper, we present direct fabrication of vertically 90-degree bent core polymer optical waveguide couplers to be integrated in optical engines enabling lens-less light coupling between laser source / photodetector and fiber with low loss. We apply the Mosquito method that we developed as a fabrication technique for single-mode and multimode polymer optical waveguides with circular cores. Because of the unique fabrication procedures of the Mosquito method, three-dimensional core alignment is possible, and thus 90° bent core can be formed directly on a laser chip. We would like to emphasize that the bending radius as small as 2 mm or less could be formed, while the insertion loss of the waveguide is as low as 5 dB.

We propose multiplexer devices in short wavelength range wavelength division multiplexing (SWDM) and mode division multiplexing (MDM) systems as applications of graded-index core Y-branched polymer waveguides. We demonstrate 25-Gbps optical signal transmissions at wavelengths of 850 nm and 1060 nm over an MMF via a Y-branched waveguide as an SWDM multiplexer (MUX) device. For an MDM MUX device, a few-mode scale asymmetric Y-branched waveguide is experimentally fabricated and the optical fields of multiple modes transmitted in the waveguide are confirmed.

We experimentally represent a 3-dimensional (3D) over-crossing single-mode polymer optical waveguides for on-board optical interconnects. This graded-index (GI) circular core 3D over-crossing structure is developed using the Mosquito method, keeping a height difference at the over-crossing section.

In this paper, we investigate the monomer dispensing conditions for the Mosquito method to accurately arrange the cores on the desired positions. Then, direct connection of waveguide core and multimode fiber is realized with the Mosquito method.

We design a measurement system to evaluate polarization dependent characteristics of polymer optical waveguides fabricated by the Mosquito method to be applied to a coupler between a single-mode fiber and silicon photonics chip. We observe the dependence of birefringence in single-mode polymer waveguides on their numerical aperture.

Polymer microchannels with circular cross-sections for on-off valves are designed and fabricated using the Mosquito method. By demonstrating liquid-flow and by applying load to the fabricated microchannels, we confirm that the microchannels appropriately close to stop flowing the liquid as an on-off valve.

In this paper, we directly fabricate vertically 90-degree bent core polymer optical waveguide right above a VCSEL chip to be applied to an optical coupler in subminiature on-board optical engine. We have proposed the Mosquito method for fabricating polymer optical waveguide utilizing a microdispenser. In the procedure of the Mosquito method, the vertically 90-degree bent cores are dispensed just above VCSEL chips by taking the advantage of the Mosquito method: ability to form three-dimensional core wiring pattern. We emphasize in this paper that the number of the vertically bent core is increased to three that are aligned in parallel and small bending radius with the 3 parallel core structure can be formed.

We propose an asymmetric polymer Y-branched waveguide for a multiplexer of TE01 and TE11 modes to a few-mode fiber in mode division multiplexing links, in this paper. Fabricated waveguides have square cross-sectional graded-index (GI) cores which exhibit strong optical confinement to the core center. Theoretical loss calculation by BPM shows that the GI core exhibits higher mode splitting ratio and about 1 dB lower insertion loss than the conventional SI core. The measurement results of the fabricated waveguide elucidate that the GI core shows an insertion loss about 1.5 dB lower than the SI core, showing a more pronounced low loss advantage. We also demonstrate in this paper that a square approximation for the core cross-section allows to reduce the coupling loss with fibers and realize a low-loss mode multiplexer, compared to the rectangular cross-sectional core.

In this paper, we fabricate GI-core multimode polymer optical waveguides with remarkably high Δ (~0.069) applying the Mosquito Method, by which the bending loss as low as 0.4 dB under 1-mm bending radius is attained.

In this paper, multimode Y-branched waveguides with low insertion losses are designed and fabricated applying our original fabrication method of polymer optical waveguides named the Mosquito Method, and then, light splitting ratios of the fabricated Y-branched waveguides are investigated for application to DEMUX devices.

We demonstrate forming a waveguide module by adhering a core in a polymer waveguide and a single-mode fiber (SMF) to stabilize the connection at high temperature. We confirm that the connections are stably maintained at 86°C.

In this article, we propose a subminiature (10.9 mm $\times56.6$ mm) vertical cavity surface-emitting laser (VCSEL)-based optical engine with a low crosstalk penalty for onboard applications. When applying optical engines to onboard interconnects, ICs [laser drivers and transimpedance amplifiers (TIAs)] and active optical devices (light sources and photodetectors) must be mounted densely to make the footprint as small as possible. It is a concern that such a high-density integration could increase the crosstalk between transmitter (Tx) and receiver (Rx) devices, which could be caused by the supply current difference between the circuit from laser drivers to light source and the circuit from photodetectors to TIAs. In this article, by inserting a gap in the ground electrode, a compact optical engine (less than half of the footprint of quad small form-factor pluggable-double density (QSFP-DD) compliant engines) enabling a 25.78-Gb/s error-free optical transmission is successfully fabricated. We optimize the gap width to decrease the crosstalk while maintaining efficient heat dissipation via the electrode. We compare the characteristics of the fabricated optical engine to the engine with the gap-less ground electrode structure formed in the same compact size. Then, we both theoretically and experimentally confirm a link power budget savings of about 1.8 dB, which is sustained even under high-temperature ( $T_{c} = 70\,\,^{\circ }\text{C}$ ) operation. In addition, to realize further high-density assembly, we also represent a lens-less optical coupling by inserting a 90o-bent graded-index (GI) core polymer waveguide between the optical transmitter and multimode fiber. The transmission performance of the 90o-bent GI-core waveguide is preliminarily evaluated, and we successfully transmit 53.125-Gb/s PAM4 optical signals experimentally.

In this paper, we propose a higher efficiency optical coupling technique using 90-degree-bent Graded-Index (GI) core polymer waveguide which replaces conventional optical assembly (mirrors and lenses) in vertical cavity surface emitting laser (VCSEL) based optical engines. The 90-degree-bent GI-core polymer waveguides are fabricated using the Mosquito method. However, high coupling loss could be caused by the VCSEL beam divergence due to diffraction in the lens free coupling. We theoretically confirm that total insertion loss including bending loss as well as coupling loss could remain low by the waveguide NA control. Fabricating the 90-degree-bent GI-core polymer waveguides with the optimized NA, a waveguide insertion loss as low as about 2 dB is experimentally achieved.